Magic Eye Tube or Tuning Indicator

When I started as a short wave listener in 1970s, I was using a Philips Prestige vacuum tube radio. It had a beautiful tuning indicator with a green display. The gap between two vertical columns of display would close while receiving a strong local broadcast station. While receiving weak stations like usual amateur radio stations, the gap would widen. For medium strong stations, when there is fading or QSB on short wave frequencies, the gap would oscillate according to the strength of the station. Gap will be lesser when the signal is stronger and vice versa, as you can see on the right side. My radio had an EM 84 valve as shown on the left side. Animation of a different type of tuning indicator tube EM 11 with circular display is shown on the right side. Both images are from Wikipedia. Read that the circular one with ‘end on view’ was called cat’s eye, or tuning eye in North America. We can find out how exactly we have tuned in to the station by looking at the gap narrowing while tuning in.